The present invention relates to industrial control systems and more particularly to remote input/output (I/O) systems for use in conjunction with industrial control devices such as programmable controllers.
Most control systems for use in automatically controlling production machinery and the like include a central control device such as a programmable logic controller which receives inputs from sensors such as limit switches and provides outputs to actuators for controlling operational machines such as conveyors. The outputs provided to the actuators are determined in accordance with a control program which is continuously executed by the controller and which utilizes the information from the input sensors in setting appropriate control signal outputs. Such systems are in widespread use and are entirely satisfactory except that it is frequently desired to receive inputs and provide outputs from groups of sensors and actuators located at remote locations. In order to avoid the use of large numbers of wires which are bulky and are difficult and costly to install and maintain, remote I/O systems are used which communicate with central control devices over local area networks. Such systems usually include a communications adapter for interfacing with the communications network and several I/O modules for handling different types of I/O signals. The use of remote I/O systems and network communications for transferring information to and from remote locations to central control points simplifies the implementation of control system applications and saves considerable effort and expense.
However, remote I/O systems themselves require flexible and efficient architectures. In particular, remote I/O systems should be designed to allow different types of I/O modules to be quickly and conveniently installed and removed and allow the systems to be rapidly and easily configured and reconfigured. Moreover, remote I/O systems must have efficient communications between the I/O modules and the communications adapter which interfaces with the local area network for transferring information to and from the central control device. Among other things, communications must be rapid and deterministic without requiring burdensome numbers of bus lines for interconnection of the I/O modules and communications adapter.
It is therefore an object of the present invention to provide a remote I/O system having a structure which allows for the rapid and convenient insertion and removal of different types of I/O modules and which allows such systems to be speedily and effortlessly configured and reconfigured.
It is another object of the present invention to provide a remote I/O system in which a common communications backplane can be easily assembled between adjacent units to provide a communications bus over which data can be exchanged between I/O modules and a communications adapter.
It is a further object of the present invention to provide a communications protocol for serial communications over a common backplane between I/O modules and a communications adapter which is highly efficient, straightforward and deterministic.
It is a further object of the present invention to provide a communications system for serially exchanging data over a common backplane between communications modules and adapters which provides for polling to indicate module presence, two-way exchange of data in single data frames and error detection to indicate the presence of corrupted data.